The name invites the wrong assumption
“Immediate-use steam sterilization” gets shortened in conversation to “flash,” and “flash” sounds like it means faster or lighter. It doesn’t. The temperature, pressure, and exposure time for a given item are set by the same physical requirements as a routine wrapped cycle — steam still has to penetrate and hold at a lethal temperature for the validated exposure time regardless of what happens before or after. IUSS doesn’t shorten that physics. What it removes is everything that normally happens around the cycle: packaging, the extended drying phase, and a storage period between sterilizer and use.
What’s actually being traded away
A routine wrapped cycle exists inside a chain: wrap or pouch, sterilize, dry, then store the sealed pack until it’s needed, with the packaging itself acting as the sterile barrier during that storage window. IUSS processes an item unwrapped (or in an open tray) with the explicit intent of using it within minutes, directly at point of use. That removes the packaging step — but packaging wasn’t decorative. It’s what lets a sterile item sit in storage without being considered compromised. Take it away, and the item is sterile only for as long as nothing touches it and nothing recontaminates it between the sterilizer door and the sterile field, with no barrier layer to fall back on if that transfer isn’t immediate and controlled.
Why standards frame it as an exception, not a default
AAMI ST79 and similar guidance describe IUSS as intended for specific circumstances — most commonly an instrument dropped or contaminated mid-procedure with no sterile replacement available — rather than a routine substitute for wrapped processing. The practical reasons line up with what’s missing structurally: no packaging means no storage assurance, transport between sterilizer and field has to be immediate and aseptic by definition, and the item typically bypasses the same labeling and lot-traceability workflow a packaged, dated load goes through. None of that makes IUSS unsafe when used for what it’s actually for. It makes it unsafe as a convenience shortcut for cases where planning ahead — running one more wrapped cycle, keeping a backup set on hand — was the available alternative.
The drying-phase question specifically
Because IUSS is by design an unwrapped, unstored process, the extended drying phase that a Class B wrapped cycle depends on (covered in the drying-phase article) isn’t the relevant concern here in the same way — there’s no pack to keep dry during storage, because there’s no storage. What replaces it is a different discipline: getting the item from chamber to sterile field without setting it down on a non-sterile surface in between, which is a workflow control, not a cycle parameter.
In practice
Any Class B autoclave capable of a standard unwrapped cycle can run IUSS — it isn’t a separate machine or a separate program in most cases, just a different load configuration and a different post-cycle handling protocol. The equipment side of this rarely needs a dedicated purchase decision; the part worth getting right is the facility policy on when IUSS is appropriate to invoke at all.